Any phospholipid is really a heterogeneous mixture of molecular species the complexity of which is dictated by the number of different fatty acids which are part of that lipid. It is not known whether there are differences in turnover rates of different molecular species or for that matter whether the saturated and unsaturated fatty acids and glycerol in the same molecular species have the same or dissimilar "apparent turnover rates." These studies are designed to measure the "apparent turnover rates" of glycerol, saturated and unsaturated acids from the choline- and ethanolamine phosphoglycerides isolated from rat liver. Subsequent studies will be designed to determine whether dietary modification alters these "apparent turnover rates." In particular we wish to determine whether dietary change alters the apparent turnover rate of 8, 11, 14-eicosatrienoic acid and arachidonic acid since these two acids are not only structural components of membrane lipids but are also precursors for prostaglandins.